Pressurized water nuclear reactors include a vessel in which the core of the nuclear reactor is located and which is closed at its top by a generally domed head. When the nuclear reactor is in service, reactor cooling fluid which consists of water at very high temperature and at very high pressure (310.degree. C. and 155 bar) flows inside the vessel, in contact with the fuel assemblies constituting the core of the reactor.
During operation of the nuclear reactor, it is necessary to measure the various physical parameters inside the core, and in particular the temperature inside certain fuel assemblies. The vessel head is traversed by tubular components termed adapters, which are arranged parallel to the vertical axis of the vessel, plumb with certain positions of fuel assemblies in the core.
On part of the adapters, multiple-part tubular supports are mounted which make it possible to ensure leaktight passage of a thermocouple column used for measuring the temperature in a group of assemblies of the core.
The thermocouple column can be engaged in the bore of the tubular support and clamped in a leaktight manner against a bearing zone, with interposition of a metal seal using a removable clamping device which bears on the upper part of the support.
To allow dismounting of the thermocouple column in any event, the tubular support is made in two parts placed end-to-end and assembled by a clamp consisting of several parts in the form of ring portions assembled by screws, bearing on frustoconical thrust bearing zones situated at the ends of the two parts of the tubular support which are attached to each other, in order to clamp the two parts of the support in the axial direction, with interposition of a seal between their abutting ends.
To facilitate handling and fitting of the clamps, which are heavy components, it has been proposed to produce these clamps in an articulated form, the clamp consisting of elements having an internal surface in the form of ring portions joined together in an articulated fashion, in the manner of a chain, by bars and by pins parallel to the axis of the clamp.
Two of the articulated elements include ends intended to close the clamp, having recesses intended to interact with the bearing components of corresponding shape when tightening the clamp.
A screw passes through the bearing components in a direction perpendicular to the axis of the clamp. Two locking nuts are engaged on the ends of the screw, so as to come into contact with the bearing components in order to tighten the clamp, by pressure on the bearing components which are themselves in contact with the recesses of the articulated elements. The bearing components each include two journals whose axes are aligned and which are introduced and held in through-openings, situated opposite each other, of two bars placed on either side of the articulated elements and of the screw which is engaged in two forked parts of the articulated elements.
One of the pairs of through-openings of the bars consists of circular openings in which the journals of one bearing component are engaged with minimal clearance, and the other pair of openings consists of oblong openings in which the journals of the other bearing component can slide in the longitudinal direction of the bars and of the screw, in order to facilitate fitting of the clamp.
After the clamp has been fitted on the junction parts of the elements of the tubular support of the penetration, the clamp is tightened by screwing one of the nuts so as to bring together the internal annular portions of the articulated elements of the clamp and to exert axial forces on the two tubular parts of the support between which a seal is interposed.
The clamps of the thermocouple column supports are subjected, during operation of the reactor, to large forces, because of the very high pressure of the reactor cooling fluid.
In the event that these forces lead to breakage of the screw for tightening the clamp, the bearing components move away from each other, which produces a degree of opening of the clamp. This relative movement of the bearing components is limited by the fact that the journals of the bearing component which are situated in the oblong openings come into abutment against the outer end of the oblong openings. The clamp remains in place on the abutting bearing parts of the supports of the thermocouple column, but no longer seal the linkage between the two parts of the support, so that an uncontrolled leak of cooling fluid occurs between the two parts of the support.
This may result in difficulties for continuing to ensure the safety of the nuclear reactor.
More generally, in the case of use of a clamp for any two tubular elements placed end-to-end and clamped together, it is highly advantageous to have a means making it possible to control a leak at the junction between the tubular elements, in the event of breakage of the screw for tightening the clamp or for retaining the locking nuts of the clamp to prevent them untightening.